Interest in using steam for cleaning semiconductor surfaces has increased in recent years due to the environmental concerns raised over the use of certain hazardous chemical solvents commonly used in the semiconductor industry. Moreover, the problem and cost of properly disposing these hazardous chemicals is becoming increasingly prohibitive. Therefore, alternatives to these hazardous chemicals have been sought.
An example of using steam to clean the surface of a semiconductor wafer is found in U.S. Pat. No. 4,186,032, issued to Ham. Here superheated steam is passed over the wafer and condensate is allowed to form and drip off the wafer.
With improved semiconductor fabrication techniques the size of the semiconductor components has continually decreased. Smaller sized components have necessitated improved cleaning methods and apparatus, since extremely small particles can interrupt a circuit. For methods using steam this has meant improving the purity of the steam used.
A common type of steam generator may be termed a batch type boiler. These boilers have the characteristic of having a feed water inlet and a steam outlet. With time impurities build up in the boiler water which can contaminate the steam. Therefore the boiler must be periodically flushed and cleaned out.
In U.S. Pat. No. 4,767,502, to Santasalo et al., a pure steam generator is disclosed which is particularly adapted for steam sterilizing products of the pharmaceutical industry. The steam generator of Santasalo et al. includes a feed water line, an electrode system for heating the feed water, a steam outlet and a droplet separator. Here a major consideration was providing a fast acting electric steam generator. The solution to this was found in enclosing the electrodes in small tubular spaces. This arrangement effectively reduces the thermal mass seen by the electrode and results in the desired rapid boiling. However, rapid boiling causes significant entrainment of droplets in the steam. To remove the droplets a long tortuous path is provided, which reduces the overall efficiency of the steam generator.
Moreover, a further loss of efficiency occurs with vigorous boiling. Theoretically once water reaches the point of boiling, all additional energy goes into converting the water into steam. In actuality the more vigorous or rapid the boiling is, the less efficient the conversion becomes. That is, with each incremental increase in the amount of power going into the conversion process the smaller the increase in the amount of steam produced. Yet large amounts of pure steam are required in most semiconductor wafer cleaning applications. The difficulty in producing large amounts of pure steam without the problem of droplet entrainment has not heretofore been adequately addressed.
In European publication no. EP-0-284-052-A2 a method and apparatus for producing ultra-pure water are disclosed. The method uses a distillation process in which the water used is first heated to remove any volatile components in the water prior to distillation. The water from which the volatile components have been removed is then heated to produce steam which is filtered by a gas-pervious and liquid-impervious membrane after which the filtered steam is condensed. A quantity of the condensed water is maintained and made available for processes requiring pure water, such as cleaning semiconductor wafer surface and medical tools.
Sometimes a problem with bacteria growth arises with maintaining or storing water, even ultra pure water, to be used for cleaning. Therefore, cleaning methods involving ultra pure water may not be completely suitable for cleaning semiconductor wafer surfaces.
It is therefore an object of the present invention to design a steam generator which efficiently produces ultra high purity steam and which may be operated for long periods of time without a significant buildup of contaminants in the generator.
It is another object of the present invention to design a generator which converts large amounts of electrical power to steam.
It is another object of this invention to provide an ultra high purity steam cleaning system for semiconductor wafers.